Typically completion of a well bore involves fracturing of the formation around the well bore to enhance or stimulate the flow of hydrocarbon (oil or gas) from the formation into the well bore. For a vertical well bore, the well bore is typically lined with steel casing, and for well completion, perforations are created in the metal casing at pre-determined depths. Horizontal or directionally-drilled wells are often completed without a steel casing at the production region. Completion of a well bore without a steel casing at the production region is known as “open hole” completion. In any case, a fracturing fluid (either with or without propping agents) is pumped at high pressures into the well bore at the production region to create a fracture into the formation for a desired fracture length. Use of a propping agent (also called proppant) in the fracturing fluid prevents the fracture from closing once pumping has ceased. The predominant fracture configuration is in the form of two wedge-like shapes oriented approximately 180 degrees from each other and extending out from the well bore. Such a configuration can be characterized by dimensions of width “W”, height “H”, and length “L”. The propped fracture provides a highly conductive conduit for the hydrocarbon to travel from the reservoir into the well bore.
Often it is desired to measure the dimensions of the fractures extending from the well bore. Measurement of the dimensions of the fractures may help diagnose post stimulation problems such as lower than expected production, and help improve future stimulation treatment designs by verifying design assumptions such as the amount of fracturing fluid and proppant that should be injected into the well bore during a fracturing operation.
For years, radioactive tracers have been used in combination with standard pulsed neutron well logging tools for measuring the fracture height near the well bore. See, for example, U.S. Pat. Nos. 3,002,091; 3,019,341; 5,413,179; and 5,441,110. Non-radioactive tracer material (such as vanadium or indium) is incorporated into coating on proppant that is injected from the tool into the fracture. The tracer material is activated by a pulse of neutrons from the tool. The tracer material then becomes temporarily radioactive, emitting characteristic gamma rays that are visible to the logging tool's spectrometer. The presence of the gamma rays permits a direct measurement of the height of the fracture near the well bore. In addition, the intensity of the gamma rays is proportional to the yield of the pulsed neutron source in the tool and the amount of tracer in the fracture (and therefore directly proportional to the concentration of the proppant in the fracture). Therefore, the radioactive tracer technique also provides an indication of the width of the fracture. The gamma rays, however, have a limited range (about eighteen inches) through the formation. Therefore, the radioactive tracer technique does not provide a direct measurement of the length of the fracture or a profile of how the length of the fracture may vary with depth.
As described in McCarthy et al. U.S. Pat. App. Pub. 2006/0102345 published May 18, 2006, and incorporated herein by reference, the geometry of a subterranean fracture is determined by introducing into the fracture a target particle and/or proppant having a dielectric constant of greater than or equal to about 2; transmitting into the fracture electromagnetic radiation having a frequency of less than or equal to about 3 gigahertz; and analyzing a reflected signal from the target particle and/or proppant to determine fracture geometry. (McCarthy, abstract.)
A logging tool is either raised or lowered so as to traverse the formation from bottom to top or from top to bottom. The logging tool is also rotated in the borehole to detect the location of the fracture. During such traversal and/or rotation, the logging tool transmits electromagnetic radiation having a frequency from 300 MHz to 100 GHz, or any portion thereof, into the formation. The electromagnetic radiation can be advantageously pulsed into the fracture from the logging tool. The receiver collects electromagnetic radiation signals from the proppants, particles, walls of the fracture or other fracture surfaces and transmits these up hole to a computer that can analyze the signals and with the help of software to develop an image of the fracture. The image of the fracture would provide data pertaining to the length and the height of the fracture (and azimuth or direction). (McCarthy, paragraph [0022].)
In another embodiment, the electromagnetic radiation can comprise spread spectrum continuous wave signals. The peak to average power ratio of continuous wave signals is low, which permits the emission of electromagnetic radiation signals down hole with a fiber optic cable and a photodiode. (McCarthy, paragraph [0066].)
As the fracture width (the separation of the walls of the fracture is termed the width) narrows, energy from the electromagnetic radiation pulse will be returned and the spectral content of the returned pulse will carry useful geometric information that will be used to characterize the fracture. Many detection schemes are possible. It may be advantageous for example to monitor the returned energy in a narrow band as a function of time (frequency domain) or accurate data may be achieved by high-resolution sampling in a very narrow time window and moving the window over subsequent pulses to map out the response (time domain). Interferometric synthetic aperture radar (SAR) techniques may also be employed to utilize valuable phase information as the antenna is moved up and down in the well bore. Finally, it may be advantageous to employ multiple frequency band pulses, using different antennas to be able to map the response over a very large frequency band. In one embodiment, the logging tool may employ more than one antenna along with the associated circuitry that allows the use of multiple frequencies to screen and to determine the geometry of the fracture. In another embodiment, the logging tool may employ a variable antenna to permit the transmission and receipt of frequencies having a large range in wavelength sizes. (McCarthy, paragraph [0067].)